1. Field of the Invention
The present invention relates to an electric discharge machining apparatus in which voltage is applied across a work gap formed between a tool electrode and a conductive workpiece to machine the workpiece. In particular, the present invention relates to a device for controlling the size of the work gap in response to feedback of voltage across the work gap.
2. Description of the Related Art
Generally, a mean voltage across a work gap (“gap voltage”) is considered to be proportional to the size or distance of the work gap. In order to maintain the size or distance of the work gap to an optimum value, the gap voltage is fed back to an NC device for electric discharge machining. The NC device controls a motor in accordance with gap voltage feedback to move the tool electrode relative to the workpiece. Wire electric discharge machines and sinker electric discharge machines are mainly known as electric discharge machines. The position of the tool electrode in a sinker electric discharge machine is controlled so as to maintain a mean value of the gap voltage (“mean gap voltage”) at a servo reference voltage. In this way, the work gap is maintained at a constant size, material continues to be removed from the workpiece by electric discharge machining, and the tool electrode is advanced towards the workpiece in the direction of a vertical Z-axis. The speed of a wire electrode in wire electric discharge machining is controlled in many cases so that the mean gap voltage is maintained at a servo reference voltage. In this manner, the size of the work gap is kept at an optimum value, and the wire electrode is moved at a controlled speed within the XY plane along a programmed path. Controlling position or speed of the tool electrode according to feedback of the gap voltage in order to maintain the size of the work gap at an optimum value is called servo control. A servo controller determines that the work gap is too narrow if a mean gap voltage is smaller than a servo reference voltage, and increases the size of the gap. On the other hand, if the mean gap voltage is larger than the servo reference voltage, control is performed to make the work gap smaller. Japanese patent application laid-open publication No. 2002-144153 discloses a wire electric discharge machine in which mean gap voltage is detected as a servo control parameter. The wire electric discharge machine of that publication is provided with a divider circuit for dividing mean gap voltage by frequency of electric discharge, and the size of the work gap is controlled according to a difference between the output of the divider circuit and a set value.
In recent years, there has been demand for a degree of surface roughness and dimensional precision of 1 μmRmax or less in electric discharge machines. U.S. Pat. No. 5,750,951 discloses a power supply for applying a high frequency alternating voltage to a work gap in order to finish a workpiece to just such a small degree of surface roughness. It is possible to repeatedly generate a current pulse having an extremely short ON time in the work gap by applying a high frequency a.c. voltage. With this type of μm order finishing, there is little waste generated from the workpiece. Even so, if microscopic machined waste induces leakage current inside the work gap, not all of the energy applied from the power supply to the work gap is put towards electric discharge generating. As a result, non-load voltage is lowered, and a current pulse of a smaller than anticipated energy may be generated, or no current pulse at all. If the mean gap voltage is also lowered due to the lowering of the non-load voltage, the servo controller performs incorrect control to increase the work gap. In this way, a sufficient amount of material is not removed from the workpiece and the desired degree of surface roughness is not obtained.